When Marina Abramovic took her place at an empty table for her performance/installation The Artist Is Present one recent day, “the first visitor in line was a young woman who showed up dressed in [Abramovic’s signature] long dark blue dress and a black braid swept over one shoulder. As Abramovic’s doppleganger, she sat across from her and assumed a mirror pose. And there she sat, to the bafflement of the museum staff and visitors, all day.” A Q&A with the perp.